Polycarbonate is an engineering plastic widely used in various applications due to its good heat resistance, impact resistance, and transparency. Polycarbonate's ability to self-char during combustion makes it especially suitable for uses in which flame retardancy is needed, for example in housing parts for electrical and electronic devices. However, in applications in which it comes into direct contact with chemicals, such as in office equipment and kitchen equipment, polycarbonate-based materials can exhibit environmental stress cracking (ESC) due to their relatively poor chemical resistance.
Incorporating thermoplastic polyesters into the polycarbonate compositions has been demonstrated to improve chemical resistance. See, e.g., European Patent No. 0 992 542 B1 to Matsumoto, and U.S. Pat. No. 6,590,015 B1 to Eckel et al. However, flame retardancy and impact strength are both compromised by the addition of thermoplastic polyester. And attempts to compensate for reduced flame retardancy by the addition of organophosphate esters yield undesirable decreases in heat resistance and impact strength. International Patent Application Publication No. WO 2012/058821 A1 of Wan et al. describes a polycarbonate-polyester molding composition incorporating an “organopolysiloxane-polycarbonate block copolymer”. That composition exhibits a desirable balance of mechanical properties, flame retardancy, and melt flow. But the composition's chemical resistance is inadequate for some applications with chemical exposure. There remains a need for polycarbonate-polyester blends exhibiting improved chemical resistance while substantially maintaining impact strength, melt flow, heat resistance, and flame retardancy.